Re: [PATCH] perf,x86: avoid missing caller address in stack traces captured in uprobe

[Peter Zijlstra <peterz@infradead.org>]

+Josj +LKML

On Mon, Jul 01, 2024 at 04:10:27PM -0700, Andrii Nakryiko wrote:
> When tracing user functions with uprobe functionality, it's common to
> install the probe (e.g., a BPF program) at the first instruction of the
> function. This is often going to be `push %rbp` instruction in function
> preamble, which means that within that function frame pointer hasn't
> been established yet. This leads to consistently missing an actual
> caller of the traced function, because perf_callchain_user() only
> records current IP (capturing traced function) and then following frame
> pointer chain (which would be caller's frame, containing the address of
> caller's caller).
> 
> So when we have target_1 -> target_2 -> target_3 call chain and we are
> tracing an entry to target_3, captured stack trace will report
> target_1 -> target_3 call chain, which is wrong and confusing.
> 
> This patch proposes a x86-64-specific heuristic to detect `push %rbp`
> instruction being traced. Given entire kernel implementation of user
> space stack trace capturing works under assumption that user space code
> was compiled with frame pointer register (%rbp) preservation, it seems
> pretty reasonable to use this instruction as a strong indicator that
> this is the entry to the function. In that case, return address is still
> pointed to by %rsp, so we fetch it and add to stack trace before
> proceeding to unwind the rest using frame pointer-based logic.
> 
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> ---
>  arch/x86/events/core.c  | 20 ++++++++++++++++++++
>  include/linux/uprobes.h |  2 ++
>  kernel/events/uprobes.c |  2 ++
>  3 files changed, 24 insertions(+)
> 
> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> index 5b0dd07b1ef1..82d5570b58ff 100644
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -2884,6 +2884,26 @@ perf_callchain_user(struct perf_callchain_entry_ctx *entry, struct pt_regs *regs
>  		return;
>  
>  	pagefault_disable();
> +
> +#ifdef CONFIG_UPROBES
> +	/*
> +	 * If we are called from uprobe handler, and we are indeed at the very
> +	 * entry to user function (which is normally a `push %rbp` instruction,
> +	 * under assumption of application being compiled with frame pointers),
> +	 * we should read return address from *regs->sp before proceeding
> +	 * to follow frame pointers, otherwise we'll skip immediate caller
> +	 * as %rbp is not yet setup.
> +	 */
> +	if (current->utask) {
> +		struct arch_uprobe *auprobe = current->utask->auprobe;
> +		u64 ret_addr;
> +
> +		if (auprobe && auprobe->insn[0] == 0x55 /* push %rbp */ &&
> +		    !__get_user(ret_addr, (const u64 __user *)regs->sp))

This u64 is wrong, perf_callchain_user() is always native size.

Additionally, I suppose you should also add a hunk to
perf_callchain_user32(), which is the compat case.

> +			perf_callchain_store(entry, ret_addr);
> +	}
> +#endif
> +
>  	while (entry->nr < entry->max_stack) {
>  		if (!valid_user_frame(fp, sizeof(frame)))
>  			break;
> diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
> index b503fafb7fb3..a270a5892ab4 100644
> --- a/include/linux/uprobes.h
> +++ b/include/linux/uprobes.h
> @@ -76,6 +76,8 @@ struct uprobe_task {
>  	struct uprobe			*active_uprobe;
>  	unsigned long			xol_vaddr;
>  
> +	struct arch_uprobe              *auprobe;
> +
>  	struct return_instance		*return_instances;
>  	unsigned int			depth;
>  };
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 99be2adedbc0..6e22e4d80f1e 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -2082,6 +2082,7 @@ static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
>  	bool need_prep = false; /* prepare return uprobe, when needed */
>  
>  	down_read(&uprobe->register_rwsem);
> +	current->utask->auprobe = &uprobe->arch;
>  	for (uc = uprobe->consumers; uc; uc = uc->next) {
>  		int rc = 0;
>  
> @@ -2096,6 +2097,7 @@ static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
>  
>  		remove &= rc;
>  	}
> +	current->utask->auprobe = NULL;
>  
>  	if (need_prep && !remove)
>  		prepare_uretprobe(uprobe, regs); /* put bp at return */
> -- 
> 2.43.0
>